This invention relates to seismic reflection data and in particular its use in determining the direction and magnitude of stresses acting within sedimentary basins in the earth's crust. The invention has a particular application in the hydrocarbon exploration and production industry and it will be convenient to hereinafter describe the invention in relation to this particular application. It should be appreciated however that the invention has wider applications.
Exploration for hydrocarbons is a risky business. There is no guarantee that having identified an area likely to contain hydrocarbons, commonly referred to as a prospect, hydrocarbons will be extracted. Hydrocarbons and in particular oil and natural gas accumulate and form reservoirs in sedimentary basins in the earth's crust. The oil and gas will desire to permeate through the sedimentary basin as a result of density and pore pressure differences and from compressional stresses generated within the earth's crust. The oil and gas will tend to rise through the sedimentary basin until stopped by a seal, such as a layer of shale, where it will accumulate and form the reservoir.
The process of successfully extracting hyrdocarbons requires an appreciation of the stresses acting across the prospect. The mutually perpendicular compressional components of the stresses acting across any prospect may be grouped into SV (vertical stress), SH (maximum horizontal stress), Sh (minimal horizontal stress). Whilst an appreciation of these stresses is required for various stages of the exploration and extraction process, appreciation is particularly important when drilling an extraction wellbore. More specifically when these stress components are not equated they tend to deform the wellbore cross section from a circle to an ellipse, which in some cases can lead to the collapse of the wellbore, a phenomenon known as wellbore breakout. However an appropriately designed wellbore can sufficiently equate the stress components to achieve extraction of the hydrocarbon. Unfortunately with current techniques an appreciation of the stress components is generally only obtained by monitoring the deformation of the wellbore. Where wellbore breakout occurs this information is clearly too late.